Today's enterprise data centers store ever-larger amounts of business critical data that must be immediately available and highly reliable. To meet reliability requirements for critical data, arrays of groups of disks are often employed. One type of array is known as a redundant array of independent disks (RAID array). Within a RAID array, data is distributed across groups of disks (“RAID groups”) in a manner consistent with the RAID level implemented, from full redundancy to parity protection. Arrays output logical units of storage (LUNs) for use by hosts; thus, the RAID distribution of the actual data across disks is transparent to the hosts. A typical RAID array exports LUNS that map to RAID protected physical storage across a group of disks. One or more spare disks may be provided in the array. If a disk in a RAID group fails, one of the spare disks is used to restore the RAID group. Once the spare disks are all used, failed disks must be replaced in order to maintain RAID protection for the LUNs associated with the group.
Applications continually evolve and continually require greater storage capacity. It has now become necessary and reasonable to provide multiple arrays of disks as storage in the data center. As always, it is important to the storage systems provider to provide highly reliable storage solutions at the lowest possible cost. When many separate arrays are employed in a system, hundreds of LUNS may be exported from many groups of disks in each array. The spare drives in a given array are eventually rendered active because of array disk failures. Furthermore, the spare drives in a given array cannot be used by the groups of disks in the other arrays. The provision of one or more spare drives per array quickly becomes inadequate. It is desirable to reduce the expense and increase the reliability of the multi-array storage solution by implementing a more efficient strategy for storage capacity sparing.